This is a minimal wrapper around jsmn.h to make things a bit easier
for iterating through a JSON object.
To use, first parse the JSON and create a contents object using
json_contents_new(). This object can then be used to initialize a
json_iter object using json_iter_init().
The json_iter object can then be parsed with json_iter_parse by
passing in JSON_MANDATORY/JSON_OPTIONAL arguments. Currently only
JSON_STRING and JSON_OBJECT types are supported. Any JSON_MANDATORY
values that are not found will result in an error.
If a JSON_OPTIONAL string is not found, the pointer will be NULL.
If a JSON_OPTIONAL object is not found, this iterator will be
initialized but 'start' will be -1. This can be checked with a
convenience macro json_object_not_found();
